Ultrahigh strength steel sheet and strength part for automobile utilizing the same

ABSTRACT

There is provided an ultrahigh strength steel sheet containing 0.10 to 0.40 mass % of C, 0.01 to 3.5 mass % of Cr, at least one selected from the group consisting of 0.10 to 2.0 mass % of Mo, 0.20 to 1.5 mass % of W, 0.002 to 1.0 mass % of V, 0.002 to 1.0 mass % of Ti and 0.005 to 1.0 mass % of Nb, 0.02 mass % or less of P and 0.01 mass % or less of S as impurities and the balance being Fe and unavoidable impurities based on the total mass of the steel sheet and having a base structure of lower bainite, a prior austenite grain size of 30 μm or smaller and a tensile strength of 980 MPa or higher. There is also provided an automotive strength part using the ultrahigh strength steel sheet.

TECHNICAL FIELD

The present invention relates to an ultrahigh strength steel sheet having good moldability and high delayed fracture resistance and an automotive strength part using the ultrahigh strength steel sheet.

BACKGROUND OF THE INVENTION

In order to achieve vehicle body weight reductions for compatibility between vehicle collision safety and environmental concern, there has recently been a growing attempt to apply ultrahigh strength steel sheets to intricate press-molded parts such as front side members, rear side members, rockers and pillars. It is thus desired to improve the moldability of the ultrahigh strength steel sheets.

Although the material strength of the ultrahigh strength steel sheets can be secured by various strengthening techniques, the workability of the ultrahigh strength steel sheets significantly decreases with increase in strength due to structural heterogeneity, local hard/soft phase coexistence and the like so that it is difficult for the ultrahigh strength steel sheets to combine both of high strength and moldability under the current circumstances. Further, the ultrahigh strength steel sheets face another problem of delayed fracture due to hydrogen embrittlement when the strength of the ultrahigh strength steel sheets becomes 1180 MPa or higher.

Against the above backdrop, attention is being given to TRIP (Transformation Induced Plasticity) steel sheets as high strength steel sheets having good moldability and showing a large elongation as a consequence of induced transformation from reduced austenite to martensite by forming deformation.

However, Non-Patent Document 1 reports that the delayed fracture of the TRIP steel sheet gets promoted by deformation induced transformation of retained austenite.

Patent Document 1 proposes a high strength steel sheet having improved delayed facture resistance by the formation of a deposit of niobium (Nb), but provides no findings about the moldability of the high strength steel sheet. There has been a demand for the ultrahigh strength steel sheets to combine both of moldability and delayed fracture resistance.

Non-Patent Document 1:

-   -   Yamazaki et al., “Effects of Retained Austenite and deformation         on Delayed Fracture Properties of Ultrahigh Strength Steel         Sheets”, Iron and Steel, 1997, Vol. 83, No. 11, P. 66-71

Patent Document 1:

-   -   Japanese Laid-Open Patent Publication No. 2005-68548

SUMMARY OF THE INVENTION

The present invention has been made to solve the above prior art problems. It is an object of the present invention to provide an ultrahigh strength steel sheet having good moldability and high delayed fracture resistance and an automotive strength part using the ultrahigh strength steel sheet.

As a result of extensive researches, it has been found by the present inventors that the above problems can be solved by forming the base structure of the steel sheet from either lower bainite, tempered lower bainite or tempered martensite and by decreasing the prior austenite grain size of the steel sheet. The present invention is based on this finding.

According to a first aspect of the present invention, there is provided an ultrahigh strength steel sheet comprising 0.10 to 0.40 mass % of C, 0.01 to 3.5 mass % of Cr, at least one selected from the group consisting of 0.10 to 2.0 mass % of Mo, 0.20 to 1.5 mass % of W, 0.002 to 1.0 mass % of V, 0.002 to 1.0 mass % of Ti and 0.005 to 1.0 mass % of Nb, 0.02 mass % or less of P and 0.01 mass % or less of S as impurities and the balance being Fe and unavoidable impurities based on the total mass of the steel sheet and having a base structure of either lower bainite, tempered lower bainite or tempered martensite, a prior austenite grain size of 30 μm or smaller and a tensile strength of 980 MPa or higher.

According to a second aspect of the present invention, there is provided an automotive strength part using the ultrahigh strength steel sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a stress-strain diagram of a plate-shaped test piece under tensile test.

FIG. 2 is a schematic diagram outlining a deep drawing test and a method of determining a limiting drawing ratio as a deep drawability factor.

FIG. 3 is a schematic diagram outlining a stretch forming test.

FIG. 4 is a schematic diagram outlining a hat bending test.

FIG. 5 is a schematic diagram showing a wall warp amount (curvature) as a shape fixability factor.

DETAILED DESCRIPTION

An ultrahigh strength steel sheet of the present invention will be first described below. In the following description, all percentages (%) in concentrations, contents, filling amounts and the like are by mass unless otherwise specified.

The ultrahigh strength steel sheet of the present invention contains molybdenum (Mo), tungsten (W), vanadium (V), titanium (Ti), niobium (Nb) or any combination thereof. Further, the ultrahigh strength steel sheet of the present invention has a base structure of lower bainite, tempered lower bainite or tempered martensite and a prior austenite grain size of 30 μm or smaller.

The steel sheet attains a tensile strength of 980 MPa or higher when the base structure of the steel sheet is formed by a hard phase of lower bainite, tempered lower bainite or tempered martensite. It is more preferable that the steel sheet has a tensile strength of 1180 MPa or higher. Generally, the tempered lower bainite phase is formed, after heating to 1100° C. or higher, under the manufacturing conditions of a finishing temperature of 850° C. or higher, a rolling draft of 30% or higher and a holding temperature of 300 to 500° C. and under the tempering conditions of a tempering temperature of 400 to 700° C. The tempered martensite phase is generally formed, after heating to 1100° C. or higher, under the manufacturing conditions of a finishing temperature of 850° C. or higher, a rolling draft of 30% or higher and a holding temperature of 150 to 300° C. and under the tempering conditions of a tempering temperature of 550 to 700° C.

The prior austenite grain size of the steel sheet is controlled to within a small grain size range of 1 to 30 μm. The steel sheet cannot expect improvements in deep drawability, stretch formability and shape fixability when the prior austenite grain size exceeds 30 μm. When the prior austenite grain size is less than 1 μm, the steel sheet is likely to deteriorate in mechanical properties and to be difficult to manufacture. It is particularly desirable to control the prior austenite grain size of the steel sheet to within 3 to 10 μm in order that the steel sheet obtains further improvements in deep drawability, stretch formability and shape fixability and thereby attains sufficient moldability required for automotive part molding.

The composition of the ultrahigh strength steel sheet will be now described below in more detail.

The ultrahigh strength steel sheet of the present invention comprises 0.10 to 0.40% carbon (C), 0.01 to 3.5% chromium (Cr), at least one selected from the group consisting of 0.10 to 2.0% molybdenum (Mo), 0.20 to 1.5% tungsten (W), 0.002 to 1.0% vanadium (V), 0.002 to 1.0% titanium (Ti) and 0.005 to 1.0% niobium (Nb), 0.02% or less phosphorus (P) and 0.01% or less sulfur (S) as impurities and the balance substantially being iron (Fe) and unavoidable impurities based on the total mass of the steel sheet. It is preferable that the ultrahigh strength steel sheet contains either one or both of 0.1 to 3.0% copper (Cu) and 0.1 to 3.0% nickel (Ni) as an additive component or components. It is also preferable that the ultrahigh strength steel sheet contains either one or both of 0.01 to 2.5% silicon (Si) and 0.1 to 1.0% manganese (Mn) as an additive component or components. Preferably, the ultrahigh strength steel sheet further contains 0.001 to 0.1% aluminum (Al) as an additive component.

With the above composition, the steel sheet is able to not only secure good moldability but also attain high delayed fracture resistance by formation of fine alloy carbide.

Carbon (C) is the most effective element for increasing the strength of the steel sheet. In order for the steel sheet to attain a strength of 980 MPa or higher, it is desirable that the C content of the steel sheet is 0.10% or higher. When the C content of the steel sheet exceeds 0.4%, however, the steel sheet is likely to decrease in toughness. The C content of the steel sheet is thus controlled to within 0.10 to 0.40%.

Chromium (Cr) is an effective element for improving the hardenability of the steel sheet and increasing the strength of the steel sheet by dissolving in cementite. It is desirable that the Cr content of the steel sheet is at least 0.01% or higher, more desirably 1% or higher. When an excessive amount of Cr is added to the steel sheet, however, it turns out that the effect of the Cr element becomes saturated and that the steel sheet decreases in toughness. The upper limit of the Cr content of the steel sheet is thus set to 3.5%.

Molybdenum (Mo) is one of the most critical elements to the ultrahigh strength steel sheet of the present invention and is effective for not only improving the hardenability of the steel sheet but also decreasing the grain size of the steel sheet by formation of alloy carbide and promoting substitution of hydrogen in the steel sheet. When the Mo content of the steel sheet is less than 0.10%, the alloy carbide is unlikely to be formed. On the other hand, Mo is an expensive alloying element. The Mo content of the steel sheet is thus controlled to within 0.1 to 2.0%.

As Tungsten (W), vanadium (V), titanium (Ti) and niobium (Nb) produce the same additive effect as Mo, it suffices that the steel sheet contains at least one element selected from Mo, W, V, Ti and Nb in order to secure not only good moldability but also high delayed fracture resistance. The W content, V content, Ti content and Nb content of the steel sheet are controlled to within 0.20 to 1.5%, 0.002 to 1.0%, 0.002 to 1.0% and 0.005 to 1.0%, respectively.

Phosphorus (P) causes a decrease in the grain boundary strength of the steel sheet. It is thus desirable to minimize the P content of the steel sheet. The upper limit of the P content of the steel sheet is set to 0.02%

Sulfur (S) also causes a decrease in the grain boundary strength of the steel sheet. It is thus desirable to minimize the S content of the steel sheet. The upper limit of the S content of the steel sheet is set to 0.01%.

Copper (Cu) is an effective element for strengthening the steel sheet and contributes to prevention of delayed fracture by fine deposit thereof. It is desirable that the Cu content of the steel sheet is 0.1% or more. However, the excessive addition of Cu results in workability deterioration. The upper limit of the Cu content of the steel sheet is thus preferably set to 3.0%.

Nickel (Ni) is an effective element for improving the hardenability of the steel sheet for sufficient steel sheet strength and increasing the corrosion resistance of the steel sheet. When the Ni content of the steel sheet is less than 1%, the Ni element does not produce a desired effect. When the Ni content of the steel sheet exceeds 3.0%, the steel sheet deteriorates in workability. It is thus desirable to control the Ni content of the steel sheet to within 0.1 to 3.0%.

Silicon (Si) is an effective element for deoxidation and strength improvement. It is desirable that the steel sheet contains 0.2% or more Si including some added as a deoxidant and remaining in the steel sheet. However, the excessive addition of Si results in toughness deterioration. The upper limit of the Si content of the steel sheet is thus preferably set to 2.5%.

Manganese (Mn) is an effective element for strength improvement. When the Mn content of the steel sheet is less than 0.1, the Mn element is unlikely to produce a desired effect. By contrast, it turns out that the cosegregation of P and S becomes promoted and that the steel sheet decreases in toughness when an excessive amount of Mn is added to the steel sheet. The Mn content of the steel sheet is thus preferably controlled to within 0.1 to 1.0%.

Aluminum (Al) is added for deoxidation. When an excessive amount of Al is added to the steel sheet, however, the amount of inclusions in the steel sheet increases to cause a deterioration in workability. The Al content of the steel sheet is thus preferably controlled to within 0.001 to 0.1%.

The ultrahigh strength steel sheet of the present invention can be processed by either hot rolling or cold rolling because of its good moldability. The thickness of the ultrahigh strength steel sheet is generally 0.5 to 2.3 mm. In view of the material design, the ultrahigh strength steel sheet may be surface treated by zinc plating or treated by film lamination.

Next, an automotive strength part of the present invention will be described below.

The automotive strength part of the present invention is produced from the above-explained ultrahigh strength steel sheet and thus combines good moldability and high delayed fracture resistance. More specifically, the automotive strength part is produced by subjecting the high strength steel sheet to any of press forming process (cold press forming, warm press forming, hot press forming), hydroform process and blow molding process.

In general, a component part has a high risk of delayed fracture due to a large residual stress when processed by cutting e.g. piercing or trimming. Even with such a cut-processed portion, the automotive strength part of the present invention has less delayed fracture and thus can be used effectively.

EXAMPLES

The present invention will be described in more detail with reference to the following examples. It should be however noted that the following examples are only illustrative and are not intended to limit the invention thereto.

Examples 1 to 5 and Comparative Examples 1 to 6

Steel sheets of Examples 1 to 5 and Comparative Examples 1 to 6 were formed from various steel materials. The compositions of the steel materials and the manufacturing conditions of the steel sheets are indicated in TABLES 1 and 2. Each of the steel sheets was tested for mechanical properties such as tensile strength and SD (stress decrease after uniform elongation), structure, moldability and delayed fracture susceptibility by the following procedures.

TABLE 1 Steel type Material composition (mass %) number C Si Mn P S Cu Ni Cr Mo V Ti Nb Al A 0.35 0.2 0.7 0.019 0.013 0.05 0.25 1 0.2 — — — — B 0.2 0.25 0.45 0.015 0.003 0.05 1 2 0.65 — — — — C 0.38 0.24 0.45 0.015 0.003 0.05 1 2 0.65 — — — 0.01 D 0.38 0.24 0.45 0.015 0.003 0.05 1 2 0.65 — — 0.03 0.01 E 0.18 1.52 0.35 0.008 0.002 0.1 0.05 3.2 0.3 — — — — F 0.07 0.545 2.415 0.008 0.002 — — 0.2 — — — — 0.038 G 0.07 0.54 2.22 0.007 0.001 — — 0.217 — — — — 0.038 H 0.18 0.2 1.82 0.009 0.002 — — — 0.02 0.05 — — — I 0.08 0.01 0.8 0.015 0.001 — — — 0.02 — 0.05 — —

TABLE 2 Manufacturing conditions Manu- Heating facturing tempera- Finishing Rolling Cooling Holding condition ture temperature draft rate temperature number (° C.) (° C.) (%) (° C./sec) (° C.) 1a 1200 920 0 30 400 2a 1200 920 35 30 400 3a 1200 920 35 30 550

1. Mechanical Properties

(1) Tensile Strength

The tensile strength was evaluated by preparing a No. 5 test piece according to JIS Z 2201 and carrying out a tensile test on the test piece according to JIS Z 2241.

(2) Stress Decrease (SD)

FIG. 1 shows a schematic stress-strain diagram of a plate-shaped test piece, such as a No. 5 test piece or No. 13 test piece according to JIS Z 2201, under tensile test. The toughness/ductility was evaluated as “good” when the test piece had a stress decrease (SD) of 180 MPa or greater on the definition of the stress decrease (SD) as a difference between tensile strength (TS) and breaking strength.

2. Structure

(1) Base Structure

The base structure was evaluated by preparing a test piece, grinding a cross section of the test piece, etching the cross section of the test piece with a nital solution and then observing the cross section of the test piece with a magnification of 100 to 1000 times by optical microscope and with a magnification of 1000 to 5000 times by scanning electron microscope.

(2) Prior Austenite Grain Size

The prior austenite grain size was evaluated according to JIS G0551. The evaluation of the prior austenite grain size was herein made on the test piece having a base structure of lower bainite.

3. Moldability

The moldability was rated in three levels: “◯ (good)”, “Δ (ordinary)” and “X (bad)” based on the deep drawability, stretch formability and shape fixability in view of the application to intricate press-molded automotive parts. The deep drawability, stretch formability and shape fixability were evaluated by the following procedures.

(1) Deep Drawability

FIG. 2 outlines a deep drawing test. In the deep drawing test, the ratio D/d_(p) between the maximum blank diameter to the punch diameter was defined as a limiting drawing ratio LDR where D was the maximum blank diameter at which cylindrical drawing was accomplished with no fracture and d_(p) was the punch diameter. Herein, a test tool unit was used including a cylindrical punch 4 of 5 mm in punch shoulder radius and 50 mm in diameter d_(p), a die 1 of 7 mm in die shoulder radius and a wrinkle suppressor 2 in such a manner as to move the punch 4 at a speed of 3 mm/sec with 50 kN of pressure applied to the wrinkle suppressor 2. The maximum blank diameter D was measured by preparing test pieces 3 from the steel sheet of each example, subjecting the test pieces 3 to deep drawing with increasing blank diameters, and then, determining the blank diameter at which the test piece was completely drawn with no fracture as the maximum blank diameter D. The limiting drawing ratio LDR was calculated as the ratio D/50 between the maximum blank diameter and the punch diameter. The deep drawability was evaluated as “good” for a larger LDR value.

(2) Stretch Formability

FIG. 3 outlines a stretch forming test. In the stretch forming test, the drawing height immediately before the occurrence of fracture during spherical-head stretch forming was defined as a limiting drawing height LDH. Herein, a test tool unit was used including a spherical-headed punch 4 of 50 mm in radius, a beaded die 1 of 5 mm in die shoulder radius and a wrinkle suppressor 2 in such a manner as to move the punch 4 at a speed of 10 mm/sec with high pressure applied to the wrinkle suppressor 2 to avoid material inflow from the surroundings. A test piece 3 of 200 mm×200 mm was prepared from the steel sheet of each example. The moving distance from the point of contact between the test piece 3 and the punch 4 to the point immediately before the fracture was measured as the limiting drawing height LDH. The stretch formability was evaluated as “good” for a larger LDH value.

(3) Shape Fixability

FIG. 4 outlines a hat bending test for evaluating a shape fixability factor. Herein, a test tool unit was used including a punch 4 of 75 mm in width and 5 mm in punch shoulder radius, a die 1 of 5 mm in die shoulder radius and a wrinkle suppressor 2 in such a manner as to move the punch 4 by 80 mm at a speed of 10 mm/sec with 200 kN of pressure applied to the wrinkle suppressor 2. A test piece 3 of 300 mm×50 mm was prepared from the steel sheet of each example. After subjecting the test piece 3 to hat bending, the test piece 3 was taken out of the test unit. The curvature of the test piece 3 was then measured in the manner shown in FIG. 5. The shape fixability was evaluated as “good” for a larger curvature value.

4. Delayed Fracture Susceptibility

The delayed fracture resistance was evaluated as “◯ (not cracked)” or “X (cracked)” by preparing a strip test piece of 100 mm×50 mm from the steel sheet of each example, bending the test piece by a hat bending test machine, unbending the test piece, subjecting a wall section of the test piece to piercing, immersing the test piece in a 0.1 mol/m³ aqueous hydrochloric acid solution for 100 hours, and then, examining the occurrence or nonoccurrence of a crack in the test piece.

The evaluation results are indicated in TABLE 3.

TABLE 3 Mechanical properties Structure Steel type - Tensile Prior γ Moldability Delayed Manufacturing strength SD Base grain size TS × LDR TS × LDH Curvature/TS Total fracture conditions (Mpa) (MPa) structure (μm) (MPa) (MPa · mm) (mm⁻¹ · MPa⁻¹) evaluation resistance Example 1 A-2a 1202 284 LB 6.3 2212 46878 2.53 × 10⁻⁶ ◯ ◯ Example 2 B-2a 1247 372 LB 5.2 2469 47635 3.06 × 10⁻⁶ ◯ ◯ Example 3 C-2a 1385 218 LB 9.5 2078 50137 3.48 × 10⁻⁶ ◯ ◯ Example 4 D-2a 1423 255 LB 7.3 2191 52366 3.14 × 10⁻⁶ ◯ ◯ Example 5 E-2a 1329 208 LB 10 2445 50143 3.42 × 10⁻⁶ ◯ ◯ Comparative C-1a 1201 69 LB 80.3 1561 37351 4.59 × 10⁻⁶ X ◯ Example 1 Comparative C-3a 1480 58 UB — 1776 44548 5.45 × 10⁻⁶ X X Example 2 Comparative F 1041 149 F — 1915 38309 4.95 × 10⁻⁶ Δ ◯ Example 3 Comparative G 832 122 F — 1514 28704 3.63 × 10⁻⁶ X ◯ Example 4 Comparative H 968 93 F — 1897 35235 5.01 × 10⁻⁶ X ◯ Example 5 Comparative I 1019 156 F — 1956 37805 1.96 × 10⁻⁶ Δ ◯ Example 6 [Note] F: Ferrite, UB: Upper bainite, LB: Lower bainite

As indicated in TABLE 3, the ultrahigh strength steel sheets of Examples 1 to 5 had a tensile strength of 980 MPa or higher and showed sufficient deep drawability, stretch formability and shape fixability to satisfy the requirements for automotive parts. No crack occurred in the ultrahigh strength steel sheets of Examples 1 to 5 in the delayed fracture test. It can be thus concluded that the ultrahigh strength steel sheets of Examples 1 to 5 combined moldability and delayed fracture resistance. By contrast, the steel sheets of Comparative Examples 1 and 2 had a tensile strength of 980 MPa but did not combine moldability and delayed fracture resistance as the prior austenite grain size of the steel sheet of Comparative Example 1 and the base structure of the steel sheet of Comparative Example 2 were out of the scope of the present invention. The base structures and compositions of the steel sheets of Comparative Examples 3 to 6 (commercial products) were out of the scope of the present invention. Some of the steel sheets of Comparative Examples 3 to 6 had a tensile strength of less than 980 MPa. The steel sheets of Comparative Examples 3 to 6 had no problem in delayed fracture resistance but were inferior in moldability to those of Examples 1 to 5.

Examples 6 to 10 and Comparative Examples 7 and 8

Steel sheets of Examples 6 to 10 and Comparative Examples 7 and 8 were formed using various steel materials of TABLE 1 under the manufacturing/tempering conditions of TABLE 4. Each of the steel sheets was tested for mechanical properties such as tensile strength and SD (stress decrease after uniform elongation), structure, moldability and delayed fracture resistance in the same manners as above. The evaluation of the prior austenite grain size was herein made on the steel sheet having a base structure of tempered lower bainite.

TABLE 4 Manufacturing conditions Manufacturing Heating Finishing Rolling Cooling Holding condition temperature temperature draft rate temperature Tempering number (° C.) (° C.) (%) (° C./sec) (° C.) conditions 1b 1200 920 0 30 400 500° C. × 1 hr 2b 1200 920 35 30 400 500° C. × 1 hr 3b 1200 920 35 30 550 500° C. × 1 hr

The evaluation results are indicated in TABLE 5.

TABLE 5 Mechanical properties Structure Steel type - Tensile Prior γ Moldability Delayed Manufacturing strength SD Base grain size TS × LDR TS × LDH Curvature/TS Total fracture conditions (Mpa) (MPa) structure (μm) (MPa) (MPa · mm) (mm⁻¹ · MPa⁻¹) evaluation resistance Example 6 A-2b 1185 280 YLB 5.9 2252 47400 2.13 × 10⁻⁶ ◯ ◯ Example 7 B-2b 1222 332 YLB 6.1 2493 48269 2.48 × 10⁻⁶ ◯ ◯ Example 8 C-2b 1337 287 YLB 8.8 2567 51033 2.65 × 10⁻⁶ ◯ ◯ Example 9 D-2b 1365 295 YLB 7.1 2648 53235 2.56 × 10⁻⁶ ◯ ◯ Example 10 E-2b 1275 223 YLB 10 2346 48106 3.12 × 10⁻⁶ ◯ ◯ Comparative C-1b 1130 93 YLB 88.5 1537 36160 4.83 × 10⁻⁶ X ◯ Example 7 Comparative C-3b 1413 69 YUB — 1766 43803 5.18 × 10⁻⁶ X X Example 8 [Note] F: Ferrite, YUB: Tempered upper bainite, YLB: Tempered lower bainite

As indicated in FIG. 5, the ultrahigh strength steel sheets of Examples 6 to 10 had a tensile strength of 980 MPa or higher and showed sufficient deep drawability, stretch formability and shape fixability to satisfy the requirements for automotive parts. No crack occurred in the ultrahigh strength steel sheets of Examples 6 to 10 in the delayed fracture test. It can be thus concluded that the ultrahigh strength steel sheets of Examples 6 to 10 combined moldability and delayed fracture resistance. By contrast, the steel sheets of Comparative Examples 7 and 8 had a tensile strength of 980 MPa but did not combine moldability and delayed fracture resistance as the prior austenite grain size of the steel sheet of Comparative Example 7 and the base structure of the steel sheet of Comparative Example 8 were out of the scope of the present invention. The above-mentioned steel sheets of Comparative Examples 3 to 6 (commercial products) were also inferior in moldability to the steel sheets of Examples 6 to 10.

Examples 11 to 15 and Comparative Example 9

Steel sheets of Examples 11 to 15 and Comparative Example 9 were formed using various steel materials of TABLE 1 under the manufacturing/tempering conditions of TABLE 6. Each of the steel sheets was tested for mechanical properties such as tensile strength and SD (stress decrease after uniform elongation), structure, moldability and delayed fracture resistance in the same manners as above. The evaluation of the prior austenite grain size was herein made on the steel sheet having a base structure of tempered martensite.

TABLE 6 Manufacturing conditions Manufacturing Heating Finishing Rolling Cooling Holding condition temperature temperature draft rate temperature Tempering number (° C.) (° C.) (%) (° C./sec) (° C.) conditions 1c 1200 920 0 30 250 600° C. × 1 hr 2c 1200 920 35 30 250 600° C. × 1 hr

The evaluation results are indicated in TABLE 7.

TABLE 7 Mechanical properties Structure Steel type - Tensile Prior γ Moldability Delayed Manufacturing strength SD Base grain size TS × LDR TS × LDH Curvature/TS Total fracture conditions (Mpa) (MPa) structure (μm) (MPa) (MPa · mm) (mm⁻¹ · MPa⁻¹) evaluation resistance Example 11 A-2c 1212 219 YM 4.9 2097 47268 1.77 × 10⁻⁶ ◯ ◯ Example 12 B-2c 1193 297 YM 4.5 2434 42948 1.63 × 10⁻⁶ ◯ ◯ Example 13 C-2c 1266 372 YM 6.9 2481 43614 1.66 × 10⁻⁶ ◯ ◯ Example 14 D-2c 1284 329 YM 7.5 2542 44940 1.62 × 10⁻⁶ ◯ ◯ Example 15 E-2c 1254 223 YM 10 2307 47313 2.53 × 10⁻⁶ ◯ ◯ Comparative C-1c 1230 263 YM 82.5 1845 39360 4.98 × 10⁻⁶ X X Example 9 [Note] F: Ferrite, YM: Tempered martensite

As indicated in FIG. 7, the ultrahigh strength steel sheets of Examples 11 to 15 had a tensile strength of 980 MPa or higher and showed sufficient deep drawability, stretch formability and shape fixability to satisfy the requirements for automotive parts. No crack occurred in the ultrahigh strength steel sheets of Examples 11 to 15 in the delayed fracture test. It can be thus concluded that the ultrahigh strength steel sheets of Examples 11 to 15 combined moldability and delayed fracture resistance. By contrast, the steel sheet of Comparative Example 9 had a tensile strength of 980 MPa but did not combine moldability and delayed fracture resistance as the prior austenite grain size of the steel sheet of Comparative Example 9 was out of the scope of the present invention. The above-mentioned steel sheets of Comparative Examples 3 to 6 (commercial products) were also inferior in moldability to the steel sheets of Examples 11 to 15.

As described above, the steel sheet has the excellent effects of attaining not only sufficient moldability required for automotive parts as compared to conventional high strength steel sheets but also improved delayed fracture resistance, while securing a tensile strength of 980 MPa, by forming the base structure of the steel sheet from either lower bainite, tempered lower bainite or tempered martensite and reducing the prior austenite grain size of the steel sheet. It is therefore possible to provide the industrially useful ultrahigh strength steel sheet having both of moldability and delayed fracture resistance and the automotive strength part using the ultrahigh strength steel sheet.

Although the present invention has been described with reference to the above specific embodiments of the invention, the present invention is not limited to the above-described embodiments. Various modifications and variations of the embodiments described above will occur to those skilled in the art in light of the above teaching. 

1. An ultrahigh strength steel sheet comprising 0.10 to 0.40 mass % of C, 0.01 to 3.5 mass % of Cr. at least one selected from the group consisting of 0.10 to 2.0 mass % of Mo, 0.20 to 1.5 mass % of W, 0.002 to 1.0 mass % of V, 0.002 to 1.0 mass % of Ti and 0.005 to 1.0 mass % of Nb, 0.02 mass % or less of P and 0.01 mass % or less of S as impurities and the balance being Fe and unavoidable impurities based on the total mass of the steel sheet and having a base structure of either lower bainite, tempered lower bainite or tempered martensite, a prior austenite grain size of 30 μm or smaller and a tensile strength of 980 MPa or higher. 2-3. (canceled)
 4. The ultrahigh strength steel sheet according to claim 1, wherein the steel sheet further comprises at least one of 0.1 to 3.0 mass % of Cu and 0.1 to 3.0 mass % of Ni based on the total mass of the steel sheet.
 5. The ultrahigh strength steel sheet according to claim 1, wherein the steel sheet further comprises at least one of 0.01 to 2.5 mass % of Si and 0.1 to 1.0 mass % of Mn based on the total mass of the steel sheet.
 6. The ultrahigh strength steel sheet according to claim 1, wherein the steel sheet further comprises 0.001 to 0.1 mass % of Al based on the total mass of the steel sheet.
 7. The ultrahigh strength steel sheet according to claim 1, wherein the steel sheet has an average prior austenite grain size of 3 to 10 μm.
 8. The ultrahigh strength steel sheet according to claim 1, wherein the steel sheet is either a hot rolled steel sheet or a cold rolled steel sheet.
 9. The ultrahigh strength steel sheet according to claim 1, wherein the steel sheet is surface treated by zinc plating.
 10. The ultrahigh strength steel sheet according to claim 1, wherein the steel sheet is treated by film lamination.
 11. An automotive strength part using the ultrahigh strength steel sheet according to claim
 1. 12. The automotive strength part according to claim 11, wherein the automotive strength part is formed by subjecting the ultrahigh strength steel sheet to any of press molding process, hydroform process and blow molding process
 13. The automotive strength part according to claim 11, wherein the automotive strength part has a cut-processed portion. 